From AMS Glossary
(Also called George's index.) A stability index that is a measure of thunderstorm potential based on temperature lapse rate, moisture content of the lower troposphere, and the vertical extent of the moist layer.
The K index is determined by the following equation:
where T is the temperature and T'd is the dewpoint in degrees Celsius at the pressure levels indicated. The higher (positive) the K index, the greater the likelihood of thunderstorm development.